There are many varieties of rheometer which are of the rotary type comprising a fixed cylinder containing the fluid and a cylindrico-conical element which is dipped into the fluid and which has a torque applied thereto. By taking measurements of the torque applied to this element and of its speed of rotation about its axis, it is possible to determine the shear stress to which the fluid in contact with the rotary element is subjected, and the shear speed of the fluid at the wall of said element, and thus to determine the viscosity of the fluid which is the ratio between the shear stress and the shear speed. It is also possible to determine the manner in which the viscosity varies as a function of variations in the shear stress applied to the fluid.
Further, by measuring the axial force it is possible to establish other rheological characteristics (in viscoelasticity, for example).
In some cases, and in particular when the fluid is blood or a biological fluid, the speed at which the moving element is rotated must be very low, for example about one revolution in several minutes, and it is thus very difficult to measure the torque and the speed of rotation of the moving element with sufficient accuracy, unless very sophisticated and very expensive apparatus is used. Such apparatus is generally reserved for use by top level specialists. Making measurements with such apparatus takes a long time and can be very difficult, in particular because the rotary element is only partially dipped into the fluid and because surface tension forces where the rotary passes through the free surface of the fluid spoil the measurements. Further, when the fluid is blood, coagulation causes a skin to form on the surface, and since the rotary element passes through this skin, measurements are also spoiled by the skin. It is generally the case that the rotary element is guided in rotation about its axis by a mechanical device, which thus has contact friction with a prortion of the rotary element, thereby further spoiling the measurements, even if only to a small extent.
Viscosity meters are also known in which the moving element is kept completely immersed in the fluid, in order to avoid some of the above-mentioned drawbacks. However, in such apparatus, speed measurements must take a very long time in order to be accurate and this gives rise to other difficulties, for example settling phenomena may take place in biological fluids.
These known viscosity meters or rheometers are thus not reliably usable by personnel having modest technical qualifications, for example by nurses or laboratory assistants in clinics, hospitals, medical analysis laboratories, etc.
Preferred embodiments of the present invention provide a high precision rheometer which avoids the drawbacks of prior apparatus and which can be used simply, quickly and safely by personnel having modest technical qualifications.